Revert "Reland "Prepare blink to unify definitions of load completion""

This reverts commit d14508aa90c8b4e15dd77d79b19087dc0a6a2c06.

Reason: Seeing a test failure (adoption-agency-crash-01.html)
- http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/34239

TBR=japhet@chromium.org
BUG=426520

Review URL: https://codereview.chromium.org/689843002

git-svn-id: svn://svn.chromium.org/blink/trunk@184611 bbb929c8-8fbe-4397-9dbb-9b2b20218538